I like my eggs left in whole slices for more flavor, you may wish to grate your hard boiled eggs as directed in this recipe. Blend vinegar, salt, sugar, mustard, pickle relish and capers.
Stir in vegetable and olive oils. Set dressing aside. Shred eggs on a medium sized grater. Cover and set aside. Clean spinach, removing tough stems. Drain on paper towels and pat dry. Place leaves in a large bowl. Saute bacon until crisp.
Remove bacon and sprinkle over spinach. Add onion and mushrooms to drippings in skillet. Saute until tender. Stir in 1 cup dressing and bring to a boil. Pour over spinach and toss. Garnish with reserved shredded eggs. Serve with additional dressing.